1. Field of the Invention
The present invention relates to high temperature fuel cell generators and secondary fuel depletion means, wherein depleted fuel and depleted air are kept separate from each other to allow treatment of depleted fuel by a special apparatus to generate and capture essentially pure carbon dioxide, thereby precluding the release of greenhouse gas to the environment.
2. Background Information
Tubular, solid oxide electrolyte fuel cell SOFC generators have been well known in the art for almost twenty years, and taught, for example by A.O. Isenberg in U.S. Pat. No. 4,395,468. There, in the main embodiment, oxygen (as present in air), as oxidant, was reacted at the inside xe2x80x9cairxe2x80x9d electrode of a closed tubular SOFC, to yield depleted air; and fuel, such as CO and H2, was reacted at an outside xe2x80x9cfuelxe2x80x9d electrode of the closed tubular SOFC to yield depleted fuel, all in a xe2x80x9cgenerating chamber,xe2x80x9d at high temperatures (that is, about 1000xc2x0 C.). The air electrode generally comprised a doped lanthanum manganite, the fuel electrode generally comprised a nickel cermet and an electrolyte disposed between the electrodes generally comprised a stabilized zirconia. The depleted air and depleted fuel were subsequently completely combusted in a separate, but attached preheating chamber, to preheat feed air. This basic SOFC generator design was carried forward, with other improvements, as shown for example in U.S. Pat. Nos. 4,664,986; 5,573,867; and 5,733,675 (Draper et al.; Zafred et al.; and Dederer et al.). Other designs have used a series of fuel cell stacks, each providing a stage containing a different electrolyte operating at a lower temperature to improve fuel gas utilization, as taught in U.S. Pat. No. 5,712,055 (Khandkar). In a somewhat similar fashion, in one embodiment of U.S. Pat. No. 5,134,043 (Nakagawa), xe2x80x9cdepleted fuelxe2x80x9d from a molter carbonate fuel cell system is sent to a separate molten carbonate anode, where the product was then mixed/contacted with oxidant/air before being introduced into the cathode section of the first molten carbonate electrolyte fuel cell. While tubular fuel cells are emphasized herein, flat/planar fuel cells, which are well known in the art, may also be used.
However, such designs could release byproducts of combustion, such as carbon dioxide into the atmosphere. Efforts are now being made on an international level to globally reduce the release of so-called xe2x80x9cgreen house gasesxe2x80x9d which includes carbon dioxide, which may contribute to global atmospheric warming. Such efforts may, indeed, lead to future legislation regarding carbon dioxide emissions from SOFCs. What is needed is a means to further treat the spent fuel from fuel cell generators to not only reduce or eliminate carbon dioxide emissions, but also to increase the capacity of the fuel cell generators to further utilize feed fuel, thereby producing more electricity. Such a need applies to both tubular and flat plate type fuel cells.
In the area of reducing carbon dioxide emissions from power plants utilizing a variety of types of fuel cells, in order to reduce the xe2x80x9cgreen house effectxe2x80x9d, U.S. Pat. No. 4,751,151 (Healy et al.) taught a carbon dioxide absorber, such as monoethanolamine, including a regenerable absorbent, for stripping carbon dioxide followed by subsequent cooling and compression. In U.S. Pat. No. 5,064,733 (Krist et al.), recognizing prior art conversion of natural gas into carbon dioxide and waterxe2x80x94with the accompanying creation of a DC electrical currentxe2x80x94in a solid oxide fuel cell, taught conversion of the carbon dioxide and water to C2H4, C2H6 and C2H2 by use of a copper, copper alloy or perovskite cathode. That cathode was in contact with the CO2, and H2O and a dual layered anode made of metallic oxide perovskite next to the electrode with an outer contacting layer of rare earth metallic oxide contacting CH4. This provided for concurrent gas phase electrocatalytic oxidative dimerization of methane at an anode on one side of a solid electrolyte and reduction of carbon dioxide to gaseous hydrocarbons at a cathode on the other side of the solid electrolyte. Other CO2 treatments include U.S. Pat. No. 5,928,806 (Olah et al.), where a regenerative fuel cell system containing two electrochemical cells in fluid communication were taught, one cell oxidizing an oxygenated hydrocarbon, such as methyl alcohcl, formic acid, etc., to CO2 and H2O and a second cell reducing CO2 and H2O to an oxygenated hydrocarbon. This produced methyl alcohol and related oxygenates directly from CO2. Also, U.S. Pat. No. 5,866,090, (Nakagaua et al.) taught treating carbon dioxide effluent, from an energy plant which uses fuel cells, with lithium zirconia at over 450xc2x0 C., to produce lithium carbonate and zirconia.
While a great many methods to treat carbon dioxide are known, a new fuel cell generator design is needed to allow segregation of the carbon dioxide for such treatment.
Therefore it is a main object of this invention to yield an improved fuel cell generator design, allowing segregation of carbon dioxide generated at the fuel electrodes.
It is a further object of this invention to yield an improved Generator design allowing ultra high fuel utilization.
These and other objects are accomplished by providing a high temperature fuel cell generator comprising a separate generator chamber containing tubular solid oxide electrolyte fuel cells, which operate on oxidant and fuel to yield depleted oxygen and depleted fuel, and a separate depleted fuel reactor chamber containing a depleted fuel reactor and operating at a potentially different temperature than the generator chamber, where all oxidant and fuel passages are separated and do not communicate directly with one another, so that fuel and oxidant remain effectively separated, and where a depleted fuel exit is provided in the depleted fuel reactor chamber for exiting a gas consisting essentially of carbon dioxide and water for further treatment, and where at least one exit is provided for depleted oxidant to exhaust to the environment.
The invention also comprises a high temperature fuel cell generator, comprising: a housing defining and separating a generator chamber, a separate depleted fuel reactor chamber, and a depleted oxidant discharge chamber; a plurality of fuel cells, each having an electrolyte contacted on one side by an air electrode and on the other side by a fuel electrode said fuel cells disposed within the generator chamber; a depleted fuel reactor disposed in the depleted fuel reactor chamber; means to flow a feed fuel gas to contact the fuel electrode of fuel cells in the generator chamber, where said fuel can react and yield partially depleted fuel gas; means to flow a feed oxidant gas to contact the air electrode of fuel cells in the generator chamber, where said oxidant can react and yield a depleted oxidant gas; means to flow partially depleted fuel gas from the generator chamber to contact the depleted fuel reactor in the depleted fuel reactor chamber, where said depleted fuel can further react and yield a completely depleted fuel gas consisting essentially of carbon dioxide and water; and means to flow oxidant gases to the depleted fuel reactor chamber to contact the depleted fuel reactor, in order to deplete fuel to near or total completion; where depleted oxidant gases are kept separated from all depleted fuel gases and said depleted oxidant gases for the generator chamber and the depleted fuel reactor chamber flow into at least one separate depleted oxidant exit.
The invention also covers a method of operating a high temperature fuel cell generator comprising a separate generator chamber, and a separate depleted fuel reactor chamber, containing a depleted fuel reactor, with at least the generator chamber containing solid oxide fuel cells containing a solid electrolyte disposed between an air electrode and a fuel electrode which operate on oxidant and fuel gases, comprising the steps: (1) feeding feed fuel gas to contact fuel electrodes of fuel cells in the generator chamber to yield partially depleted fuel gas; (2) feeding partially depleted fuel gas to contact the depleted fuel reactor to yield almost completely depleted fuel gas consisting essentially of carbon dioxide and water; and (3) feeding oxidant gas to contact air electrodes of fuel cells in the generator chamber to yield depleted oxidant gas; where depleted oxidant gases are kept separated from all depleted fuel gases and said depleted oxidant gases flow into a separate depleted oxidant discharge chamber. In the operation of this generator, the depleted fuel reactor chamber may be operated at a temperature different than that of the generator chamber.
Thus, this invention relates to an arrangement of components within an fuel cell generator by means of which the exhausted CO2 and H2O are segregated from exhausted vitiated air. By this means water can be condensed from the product exhaust stream, and the carbon dioxide can be pressurized or liquefied and put to use, rather than being released to the atmosphere. Consequently the release of a xe2x80x9cgreen house gasxe2x80x9d pollutant is avoided. The main idea is to produce electric power and sequester carbon dioxide. In addition to the main generating section, the arrangement allows that the fuel depletion means, a depleted fuel reactor, can be a second set of power producing fuel cells designed to operate at very high fuel utilization without the normally attendant problem of oxidation of the fuel electrodes. In the arrangement of this invention the main generator chamber and the depleted fuel reactor chamber are conveniently integrated within a common enclosure. The common enclosure also contains a steam plenum, as well as stack reformer passages positioned between the fuel cells in the generating chamber.
Also included as part of the invention is a fuel cell generator where a barrier is interposed between a spent fuel plenum and vitiated air plena, preventing the migration of depleted fuel into vitiated air (or vitiated air into depleted fuel). The fuel cell generator also includes means to re-circulate depleted fuel and mix it with fresh fuel by use of an ejector pump, a mechanical pump, or other means of forcing circulation. Preferably, the generator chamber and the depleted fuel reactor chamber are maintained at different temperature levels through the regulation of the following parameters: air flow rate, air inlet temperature, fuel flow rate, and electric current drawn from the fuel cells.
The fuel cell generator advantageously can have fuel cells used in the generator chamber and in the depleted fuel reactor chamber, constructed using different anode materials (for example, nickel anodes in the generator chamber and copper anodes in the depleted fuel reactor chamber), with the temperatures in the respective chambers controlled to appropriate levels (for example, 1000xc2x0 C. in the generator chamber and 800xc2x0 C. in the depleted fuel reactor chamber) through separatexe2x80x94but coordinatedxe2x80x94regulation of the following parameters: air flow rate and/or air temperature at inlet to the generator chamber and the depleted fuel reactor chamber. Additionally, one possible design includes having the generator chamber and the depleted fuel reactor chamber separated, and not in the same exterior housing, but in physically separate enclosures suitably connected by piping.
Also included as part if this invention is a method of operating a fuel cell generator where the fuel cells in the depleted fuel reactor are effectively short circuited by means of a shorting conducting path or by means of an electrolyte with electronic as well as ionic conductivity, with the fuel utilization within the generator chamber modulated and/or the temperature of the depleted fuel reaction chamber modulated via a control system in conjunction with an oxygen sensor at the depleted fuel reactor""s exit, to monitor exit oxygen partial pressure and/or the measurement of the voltage of depleted fuel reactor chamber""s cells so as to preclude oxidating of depleted fuel reactor chamber cell anodes. Additionally, in the method, the cell voltages of the fuel cells in the depleted fuel reactor are monitored with this information used by a control system to modulate the depleted fuel reaction chamber fuel cell current to avoid anode oxidation.